Dear lammps users,
Could you tell me what distance units LAMMPS uses in pair_style and pair_coeff?
For example, I use the following command
pair_style lj/cut 5.0
pair_coeff * * 1.0 1.0
But there are two kinds of distance units: box units, and lattice units.
Which units is used for pair_style/coeff command?
In region command, we add “box units” following the command to use box units.
Therefore, should I make a command such as “pair_style lj/cut 5.0 box unuits”?
Best regards,
Yuki
Please correct me if I am wrong, but most likely, it follows the set of units you have chosen with the command units in your input script.
The specific syntax for the pair_coeff command is described in the documentation of the individual pair_style command. That documentation also contains a list of the Para and their meaning and what specific unit (e.g. length or energy) applies. You may only add “units box” in commands where it is mentioned (please also note order and spelling). If you add them where it is not supported you usually get an error, in a few cases it may be ignored.
Yes, this is correct.
Thank you for answers and comments.
My question comes from my old question about the velocity.
The following is the answer of my old question.
the velocity command applies input by default in multiples of the lattice spacing in that direction (units lattice ) and those can be different from the actual units (units box ) unless you have a default lattice setting.
Based on the above answer, I considered that the value of cutoff in input file is normalized by the lattice spacing.
It is a mistake to make assumptions about the syntax of some command based on other commands.
The canonical source for which keywords can be used where and what meaning they have is the manual. Since LAMMPS is a code with many contributors, and since each contributor has different preferences, you must not assume that all commands behave exactly the same and accept exactly the same names, conventions and arguments for the same purpose everywhere.
While the LAMMPS developers strive for consistency, it is not always possible or desirable or simple enough to keep things the same.
Keywords like the “units” keyword are only used where there is (or has been) a significant degree of convenience to express properties in multiples of the lattice spacings. Since those may be different in x-, y-, and z-direction, they are obviously ambiguous for any non-directional properties like radii or distances.